Air Masses and Fronts

Air Mass

• A large body of air in which there are similar horizontal temperature and moisture properties.

• Properties are largely acquired from the underlying surface

Air Mass

• Air mass over cold ground

Cold and dry

• Air mass over water

More moist

How does water temp affect moisture?

Air Mass Classification

• Air masses are classified according to their temperature and moisture characteristics– “continental” = dry (c)– “maritime” = wet (m)– “polar” = cold (p)– “tropical” = warm (t)– “arctic” = frigid (a)

• These are combined to create categories

Air mass classification

• mT = maritime tropical– warm/moist; originate over tropical oceans

• cT = continental tropical– warm/dry; originate over areas like SW U.S.

• mP = maritime polar– cold/moist; originate over polar oceans

• cP = continental polar– cold/dry; originate over interior continents in winter

• cA = continental arctic– frigid/dry; form at very high latitudes

Source RegionsFigure from apollo.lsc.vsc.edu/classes/met130

Fronts

• “Boundary between different air masses”

• Types of fronts– Cold– Warm– Stationary– Occluded

Maritime Polar (mP)

• Forms over the oceans at high latitudes

• Moist• Cold • Can contribute to

significant snowfall events in mid-Atlantic

• Figure from apollo.lsc.vsc.edu/classes/met130

Continental Polar (cP)

• Forms over the northern continental interior (e.g., Canada, Alaska)

• Long, clear nights allows for substantial radiational cooling (stability?)

• Assisted by snowpack• Dry• Cold

• Figure from apollo.lsc.vsc.edu/classes/met130

Arctic (A,cA)

• Similar to cP, but forms over very high latitudes (arctic circle)

• Dry• Extremely cold

• Figure from apollo.lsc.vsc.edu/classes/met130

Continental Tropical (cT)

• Forms over southwest U.S. & Northern Mexico

• Source region includes west Texas

• Dry• Warm• Limited water bodies and

vegetation limits effect of evaporation and transpiration

• Figure from apollo.lsc.vsc.edu/classes/met130

Maritime Tropical (mT)

• Forms over Gulf of Mexico as well as subtropical Atlantic and Pacific Oceans

• Moist• Warm

• Figure from apollo.lsc.vsc.edu/classes/met130

Air Mass Modification

• Air masses can be modified once they leave their source region.

• Temperature & moisture content can increase or decrease

• So how are air masses modified?

Air Mass ModificationFigure from ww2010.atmos.uiuc.edu

• 1. Move over warmer or colder ground

Air Mass Modification Figure from ww2010.atmos.uiuc.edu

• 2. Move over a large body of water

Fig. 9-12, p. 264

Box 9-2, p. 263

Example: Lake Effect Snow

Air Mass ModificationFigure from www.usatoday.com/weather/wdnslope.htm

• 3. Move over a mountain range

Air Mass Modification

• Stability of the air mass can also modified

Fronts

• Air masses move from source region through advection

• Air masses do not readily mix together

• Front – A boundary between two different air masses

• Can be hundreds of miles long

Types of Fronts

• Cold Front

• Warm Front

• Stationary Front

• Occluded Front

Cold Front

• Cold air advances, replaces warm air at the surface

• Change in wind direction/speed

• Minimum in atmospheric pressure

Fig. 9-14, p. 266

Cold Front Cross Section

• A front is a 3-D boundary• Front slopes back over the cold air mass• Warm, less dense air is lifted• Clouds/precipitation associated with a front depend

on stability and moisture• Sharp vertical motion at cold front can force

thunderstorm activity

Fig. 9-15, p. 266

Fig. 9-16, p. 267

Slope of a Front

• Depends on temperature and wind differences between the two air masses

• Shallow vs. steep slope

Warm Front

• Warm air advances• Replaces the cold air

at the surface• Change in wind

direction/speed

Fig. 9-17, p. 268

Warm Front Cross Section

• Front slopes back over the cold air mass• Slope is more gentle than with a cold front (less thunderstorm

activity)• Warm, less dense air lifted over the cold air (called

overrunning)• Clouds/precipitation depend on moisture and stability, usually

follow a set progression with an increase in altitude• Responsible for a lot of hazardous winter weather

Fig. 9-18, p. 269

Fig. 9-19, p. 270

Stationary Front

• Air masses at surface do not move, so the front is stationary

• Overrunning still occurring, so we often still see cloudiness

• Figure from ww2010.atmos.uiuc.edu

Occluded Front

• Separates cool air from relatively colder air at the surface

• Sometimes thought of as the “cold front catching up to warm front”

• The warm air mass is found above the ground

• Two types:– Cold-type occluded front– Warm-type occluded front

• Figure from ww2010.atmos.uiuc.edu

Development of Occluded FrontFigures from ww2010.atmos.uiuc.edu

Cross Section of Occluded Front

Fig. 9-20, p. 271

Occluded Front

Dryline

• Dry air (lower dewpoint temperatures) found to west, moist air (higher dewpoint temperatures) found to east

• Temperature change is rather limited across the boundary

• Common in the southern plains during the spring• It is a convergence line for wind at the surface, and is

therefore responsible for initiating many of our tornadic thunderstorms in the south Plains

• Motion is tied strongly to insolation, and typically exhibits a diurnal “sloshing” motion (moving eastward during the day, westward at night)

Fig. 9-21, p. 272

Air Masses with the Drylinewww.geog.umn.edu/faculty/klink/geog1425/images/front/dryline_airmass.jpg

Surface Dew Points

Animation

• Satellite

• WTM

http://www.cira.colostate.edu/ramm/newgoes/dust.jpg

Fronts

• “Boundary between different air masses”

• Types of fronts– Cold– Warm– Stationary– Occluded

Look for sharp changes in:

a) temperatureb) dew pointc) wind directiond) pressure and e) cloud/precipitation patterns.

Identification of Fronts on a Weather Map

Cold Fronts

• Divides cold/dry air (usually a cP air mass) from a warm/moist air mass (mT)

• Cold air is advancing on the warm air• Cold air is denser, pushes warm air up and over• May result in heavy localized precipitation ahead

of the front• Usually trails down and to the south of a mid-

latitude cyclone

Cold Front Transition

Cirriform clouds spreadahead of front

Most precipitation and deepclouds form ahead of front

Warm air rises in a steep fashion over intruding cold air

cP Air mass

mT airmass

Where’s the Cold Front?

Warm Fronts

• Boundary between warm/moist air (mT) and cool/moist or dry air (cP or mP)

• Warm air is advancing on the cold air

• Since warm air is less dense, it overruns the cold air and rises in a more gentle slope

• Usually results in more widespread and light precipitation

Note the progression of cloud typesone would observe as the warm front approaches

Stationary Fronts

• A front that is not moving

• Boundary between two air masses, usually warm and cold

• May not be associated with a mid-latitude cyclone

• However, mid-latitude cyclones may develop along stationary fronts

Occluded Fronts

• Formed when a faster moving cold front “catches up” to the slower moving warm front

• The warm mT air mass is then pinched up away from the surface

• Thus, occluded fronts usually divide cold air (behind the cold front) from cool air (ahead of the warm front)

• Usually occur in the dying stages of mid-latitude cyclones

Drylines

• Not a traditional front, but are important

• Divide moist air mass from dry air mass– both have similar temperatures

• dry air is more dense than moist air, tends to create steep rising motion along dryline– results in thunderstorms, sometimes severe